1. Field of Invention
The present invention relates to compositions of alginate nanocapsules and methods for preparing thereof, and more particularly, relates to compositions and methods for preparing alginate nanocapsules as carriers for the delivery of drugs and macromolecules in non-parenteral, parenteral, oral, and inhalation therapy as well as tools in other biomedical applications.
2. Description of Related Arts
Alginate is used extensively as a mold-making material in dentistry and prosthetics, and in textiles. It is also used in the food industry for thickening soups and jellies. Chemically, alginate is a type of polysaccharides isolated from brown algae. It is a linear copolymer with homopolymeric blocks of guluronic (G) and mannuronic (M) acids as G blocks, M blocks and guluronic-mannuronic (G-M) alternating sequences. Among these three types of structural elements, only G blocks are involved in the gelation of alginate by reacting with multiple cations such as Ca2+ and Ba2+.
The monomers can appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks), alternating M and G-residues (G-M blocks) or randomly organized blocks. The relative amount of each block type varies with the origin of the alginate. Alternating blocks form the most flexible chains and are more soluble at lower pH than the other blocks. G-blocks form stiff chain elements, and two G-blocks of more than 6 residues each can form ionically cross-linked junctions with divalent cations e.g. Ca2+, Ba2+, Sr2+ among others, leading to a three-dimensional gel network. In these ionically cross-linked gels, it is mostly the homopolymeric G blocks that form the junctions, where the stability of the gel is determined by the relative amount of divalent cations combined.
There exists a free carboxyl group on each of the guluronic or mannuronic moiety. When these carboxyl groups are not ionized, e.g. in a low pH aqueous environment, alginate molecules are not hydrated and become insoluble. However, alginate molecules become soluble and fully hydrated when the pH is neutral or alkaline. Under this condition, the reaction between G blocks and cations such as Ca2+ and Ba2+ leads to the ionic gelation of alginate. Encapsulation or containment of active ingredients is realized once alginate polymers complete phase transition from a liquid to a solid gel.
U.S. Pat. No. 4,352,883 discloses the use of alginate in preparing semipermeable capsules for the encapsulation of biological materials such as pancreatic islets and mammalian cells. Microcapsules served to isolate the cells from immunological responses of the host while transplanted tissues or cells secreting biological molecules in order to correct abnormal physiological conditions as the case of insulin secreted by β cells in the metabolic disease of Type I diabetes. Shen et al. developed fluidized-bed perfusion bioreactors with hybridoma cells encapsulated in calcium alginate (Cytotechnology, 14:109-117, 1994) and a composite gel beads (Cytotechnology 16: 51-58, 1994) as an effort to increase cell densities and productivities of bioreactors manufacturing biologicals such as monoclonal antibodies. However, these applications required capsules relatively large, 100-2,500 μm in diameter, for reasons either the sizes of tissues to be encapsulated or the operational requirement of the bioreactor systems.
Abraham et al. (Pharmaceutical Development & Technology 1(1): 63-68, 1996) used the Turbotek Atomization technique in a closed system and generated calcium alginate beads of 2.6-7.8 (NWMD) or 4.8-17.2 μm (VWMD) in diameter.
U.S. Pat. No. 4,822,534 discloses a method for producing alginate microspheres of 20-800 μm in diameter by introducing insoluble calcium salt (citrate) to alginate, emulsifying in a hydrophobic liquid and adding oil soluble acid (acetic acid) to allow calcium ions to be released and cause the alginate to gel. Monshipouri and Price used a similar strategy and produced calcium alginate capsules of 50-200 μm in diameter (J. Microencapsulation 12 (3): 255-262, 1995).
Wen et al. emulsified alginate solution in isooctane (J. Microencapsulation 9 (3): 309-316, 1992) first and then added calcium chloride solution to the emulsion. The calcium alginate capsules thus produced were up to 150 μm in diameter. In a similar way, U.S. Pat. No. 5,674,495 claims an oral vaccine composition of antigens in calcium alginate capsules, sizing from 1-30 μm in diameter, prepared by adding calcium chloride solution to emulsion of alginate containing antigens.
U.S. Pat. No. 1,432,064 discloses a process of making calcium alginate by reacting emulsion of alginate with the microemulsion of calcium chloride, to produce capsules of alginate. However, there is no information disclosed about the size and physical properties of gels.
Conclusively, to encapsulate biologically active ingredients within alginate capsules, it is critical to realize the completion of the phase transformation of alginate from a liquid to a solid gel. Conventionally, such phase transformation is processed by reacting liquid droplets of alginate made by extrusion, atomization or emulsion with the ionic crosslinking solution or emulsion of ionic crosslinking solution such as calcium chloride. All these methods used the same mechanism that the moieties of alginate contact directly with ionic crosslinking solution or emulsions of ionic crosslinking solution. However, the ionic crosslinking solution or emulsions of ionic crosslinking solution would be rather costly in practices.
On the other hand, in conventional emulsion approach, gelation of alginate starts after the collision and fusion between the droplets of alginate and ionic crosslinking solution. Thus it is unavoidable to introduce cavities on the alginate capsules formed or cause variations of the alginate capsules from spherical shape such as hemispherical particles.
As a result, to effectively deliver biologically active ingredients, it is believed that there is a need to develop new method to prepare alginate capsules smaller than 1 μm, wherein the complicated crosslinking procedure could be optionally chosen in applications.